This is my top pick. Not only meets the challenge unlike a lot on here, the gaping mouth shows great excitement and a thirst for playfulness. I also love the interaction between child and dog, which was one of the challenge descriptions.

One nit pick is the processing, looks like you shot this in bright daylight, hence the PP, I would of loved to have seen you use a larger aperture to blur out the background more, but who knows maybe it was a candid, and there was no time for that.